Optical thin films are important in the technology of coated optical surfaces. Optical coatings cause modifications in the transmitted and reflected intensities of light from interference that occurs when two or more beams of light are superimposed. If a film of a transparent substance having an appropriate thickness and refractive index is deposited on a lens, for example, the reflection of particular wavelengths of light from the lens surface can be almost completely suppressed. The light that otherwise would be reflected is not absorbed by such an antireflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a corresponding increase in the intensity of the light that is transmitted. The beneficial effects of thin film coatings, such as antireflection, are so desirable that substantially all high quality optical components are provided with optical coatings.
Some advanced applications of optical technology have performance requirements that exceed the capabilities of conventional multiple layer thin films. New optical design procedures have been developed for these advanced applications to predict the continuous refractive index profile required for any desired transmission or reflection spectrum. These design techniques employ gradient index films, in which the index of refraction varies continuously as a function of depth into the film. Gradient index optical coatings have advantages over conventional technologies, including flexibility in filter design and increased stability in adverse environments. For example, the absence of discrete interfaces is predicted to lead to greater resistance to laser damage.
One type of gradient index structure is a rugate filter, the simplest manifestation of which has a periodic refractive index that varies sinusoidally with respect to optical thickness. A rugate filter is a gradient index analog of a quarterwave stack reflector. Compared to a quarterwave stack, however, a rugate filter has greatly suppressed high-frequency reflection harmonics. The rugate structure provides high reflectivity within a narrow bandwidth simply by increasing the number of periods in the filter.
Practical realizations of the rugate and other gradient index structures have been inhibited by the limitations of thin film fabrication technology. These limitations make it difficult to ensure that a fabricated coating accurately implements the theoretically specified refractive index profile. One prior method described in U.S. Pat. No. 4,707,611, which is incorporated herein by reference, measures the reflectance of two different wavelengths of light to determine the thickness and refractive index of an incremental thin film layer deposited on a base stack of layers. However, when a coating specification calls for a continuous refractive index profile, the thickness monitoring techniques of the prior art do not provide sufficient accuracy to ensure that the deposited layers will conform reliably to the specified profile. A slight error in the deposition thickness of a portion of a rugate filter, for example, can introduce a phase shift that may have a significant detrimental effect on the filter spectral structure. Also, an error in the refractive index of such a filter will add additional frequency components to the spectral profile, resulting in the generation of unwanted sidebands in the transmittance or reflectance spectrum. It is very difficult to compensate for such perturbations by any changes in the deposition of the remaining portion of the filter. Consequently, there is a need in the art for a method of directly monitoring and controlling the refractive index of a continuous gradient-index optical film while it is being deposited.